A wide variety of implantable therapy delivery devices have been developed including pacemakers, cardioverter/defibrillators, and cardiomyostimulators. For many individuals with heart disease, these devices provide the best and sometimes the only therapy to restore the individuals to a more healthful condition and a fuller life.
Pacemakers, for example, are typically designed to operate using various different response methodologies, such as, for example, nonsynchronous or asynchronous (fixed rate), inhibited (stimulus generated in the absence of a specified cardiac activity), or triggered (stimulus delivered in response to a specific hemodynamic parameter). Generally, inhibited and triggered pacemakers may be grouped as “demand”-type pacemakers, in which a pacing pulse is only generated when demanded by the heart. To determine when pacing is required by the pacemaker, demand pacemakers may sense various conditions such as heart rate, physical exertion, temperature, and the like. Moreover, pacemaker implementations range from the simple fixed rate, single chamber device that provides pacing with no sensing function, to highly complex models that provide fully-automatic dual chamber pacing and sensing functions. For example, such multiple chamber pacemakers are described in U.S. Pat. No. 4,928,688 to Mower entitled “Method and Apparatus for Treating Hemodynamic Dysfunction,” issued May 29, 1990; U.S. Pat. No. 5,792,203 to Schroeppel entitled “Universal Programmable Cardiac Stimulation Device,” issued Aug. 11, 1998; U.S. Pat. No. 5,893,882 to Peterson et al. entitled “Method and Apparatus for Diagnosis and Treatment of Arrhythmias,” issued Apr. 13, 1999; and U.S. Pat. No. 6,081,748 to Struble et al. entitled “Multiple Channel, Sequential Cardiac Pacing Systems,” issued Jun. 27, 2000.
Pacemakers include cardiac lead electrodes for delivering cardiac therapy. Generally, such electrodes are used to stimulate cardiac tissue with electrical impulses having amplitudes ranging in volts, e.g., from about 1 volt to about 10 volts. Most “demand”-type pacemakers also include sense amplifier circuitry. Such circuitry generally includes sense amplifiers for recording and/or deriving sensed cardiac electrical activity. Generally, such amplifiers are low current, low voltage devices and are used to sense heart cardiac signals typically having amplitudes ranging in millivolts, e.g., from about 1 millivolt to about 20 millivolts. The sense amplifiers are used to control the delivery of therapy in accordance with a predefined algorithm. As such, pacemakers may be (i) prompted to generate electrical stimulating pulses if a heart needs therapy or (ii) inhibited from generating unnecessary output electrical stimulating pulses if a heart is functioning properly. Dual-chamber cardiac pacemakers, for example, typically have separate sense amplifiers for atrial and ventricular sensing. The sense amplifiers detect the presence of intrinsic signals, such as P-waves occurring naturally in the atrium and R-waves occurring naturally in the ventricle. As mentioned, upon detecting intrinsic signals from the heart, the sense amplifier circuitry generates a digital signal (for output to other components), which can either prompt or inhibit the delivery of a pacing pulse to the corresponding chamber via the electrodes.
In the case where a pacing pulse is delivered to cardiac tissue, immediately following such delivery, a residual pace polarization artifact (also called a post-pace polarization artifact or a pace polarization signal) is typically generated. Such an artifact is generally some fraction of the pacing pulse. With respect to impedance sensed by the device's internal circuitry, the total load of the pacing circuit comprises the impedance of the lead itself, the electrode-tissue interface impedances, and the impedance of the body tissue bulk. The impedances of the body tissue and the lead may be modeled as a simple series bulk resistance, leaving the electrode-tissue interface as the capacitive energy absorbing/discharging element of the total load. As such, the artifact generated by the pacing pulse is temporarily captured at the interface between pacing electrode and cardiac tissue. Subsequently, the energy of the pace polarization artifact discharges, creating an after-potential. Generally, the tip and ring electrodes serve as storage elements for the after-potential; however, the tip electrode is the primary after-potential storage element in comparison to the ring electrode.
Subsequently, if the pacing pulse captures the heart and causes an evoked response in the cardiac tissue, the evoked response signal is superimposed atop the typically much larger amplitude pace polarization artifact. As a result, conventional pacemakers or pacemaker-cardioverter/defibrillators (“PCD's”) either cannot differentiate, or have difficulty differentiating, between pace polarization artifacts and evoked response signals. This problem is further complicated by the fact that residual pace polarization artifacts typically have high amplitudes even when evoked response signals do occur. Consequently, it becomes difficult, if not impossible, to detect an evoked response signal using a conventional pacemaker or PCD sense amplifier employing linear frequency filtering techniques. As a result, many pacemakers cannot effectively discern between pace polarization artifacts and evoked response signals.
Pacemakers have been employed to use sensing and timing circuits that do not attempt to detect evoked response signals until the pace polarization artifact is no longer present or has subsided to some minimal amplitude level; only then is sensing considered reliable. This is generally due to the limited dynamic range of the sensing amplifiers. With respect to capture detection, in which the pacemaker detects whether the pacing pulse to the cardiac tissue evoked an effective stimulated response, such sensing typically occurs a significant period of time after the evoked response signal has already occurred. As a result, such pacemakers may not accurately detect evoked response signals.
Pacemakers have also been employed to minimize the pace polarization artifacts by maintaining some sort of charge balance. These designs typically involve using passive charge circuitry (e.g., analog circuitry) to minimize the artifact from the electrode. However, even by minimizing the pace polarization artifact in this fashion, an artifact may still remain that is beyond the millivolt dynamic range of the sense amplifier so as to make the evoked response difficult to differentiate. Further, the charge balance using such circuitry is often gradually achieved (e.g., in hundreds of milliseconds), increasing the likelihood that the evoked response, which can occur quickly after the stimulus signal during tachycardia or fibrillation episodes (e.g., within 5 to 20 milliseconds after the stimulus signal), may be missed.
In summary, when providing cardiac therapy using implantable therapy delivery devices, the generation and delivery of an electrical pulse to the heart gives rise to charge in the electrode-tissue interface. Such charge leads to the creation of pace polarization artifacts, which typically have much larger amplitudes than those corresponding to electrical signals arising from an intrinsic heartbeat or a stimulated response. In turn, the pace polarization artifacts can interfere with the detection and analysis of an evoked response to a pacing pulse. Methods have been developed to address this problem, all of which generally have shortcomings. Thus, a need exists in the medical arts for a system, which reliably senses evoked response signals in a pacing environment so as to overcome the problems mentioned above, among others.